Among the techniques used to deposit thin films, chemical vapor deposition (“CVD” hereinafter) is a method that exposes substrates to one or more reacting gases. The non-volatile decomposition product is a solid material that accumulates on the substrate in the form of a thin film.
A variant of CVD is atomic layer deposition (“ALD” as previously noted). In this technique, surfaces are exposed to gaseous species that exhibit limited reactivity, that is, that the reaction and therefore the growth shuts itself down after all surface reacting sites are consumed. Conventional ALD is carried out by sequentially exposing surfaces to two or more different gaseous species. The growth during each exposure is self-limited, and this leads to homogeneous films.
In a more recent innovation of the ALD method, these sequential exposures mentioned above are separated not in time, but in space. In one configuration, a surface is moved across different enclosures, each of them containing a different gaseous species. In a second configuration, the surface is at rest, and a movable head containing two or more different enclosures is moved over the surface, resulting in alternate exposures to the reacting gases. Combinations of these two configurations also form part of the prior art.
All these prior art designs share a common feature: the need to physically isolate one gas from the other to avoid gas-phase reactions between the two species; and this feature would effectively kill the self-limited nature of ALD. Since there must be relative motion between the surface and the enclosure wherein the gas is dosed, it is not possible to have an airtight sealing of the gaseous species. Instead, a fraction of the species will diffuse out of the enclosure through the space between the moving substrate and the enclosure walls. Moreover, both the excess gas that does not react with the surface and the gas that escapes from the enclosure need to be removed from the system. This methodology requires the implementation of different purge strategies, including purge regions separating enclosures with two different kinds of gases to quickly remove the gas before it leads to undesired gas phase reactions or the use of high pressure gas to act as effective barriers between the different enclosures. Therefore, such prior art systems and methods require construction of complicated structures and highly demanding chemical processing conditions which make difficult and impractical the use of ALD for most commercial applications.